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Proving dead: three tests, or ten, and never trusting someone else's isolationThree phase. How many tests to prove it dead?

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Proving dead: three tests, or ten

Work from the least dangerous conductor first. Your CPC is the least dangerous, so that's where you start, and you move one probe at a time — it's quicker and it keeps a hand still.

3Single phase
1 · CPC to neutral
2 · CPC to line
3 · line to neutral
10Three phase
Four to earth, three to neutral, three between lines
#Three-phase sequence
1–4CPC to neutral · CPC to L1 · CPC to L2 · CPC to L3
5–7Neutral to L1 · neutral to L2 · neutral to L3
8–10L1 to L2 · L1 to L3 · L2 to L3
The last three are the most dangerous, which is exactly why they're last. Neutral is less dangerous than line; line to line is the worst case. Work up the list, not down it.

Where the probes go — and it differs by system

SystemWhere you testWhy
Single phaseLoad (busbar) side of the main switch — usually the bottom on a consumer unitThe busbar links all the breakers and RCDs, so the load side of the main switch proves every circuit
Three phaseLoad (busbar) side of the main switch, or the busbar chamberWhich end the supply comes in varies between boards: find the busbar side on the board in front of you
A busbar chamber is what you'll meet on larger three-phase installations — big copper bars with a brown, black and grey cable landing on them. That's a perfectly valid place to prove dead.
Does the lead colour matter? Not on safe isolation, and not on dead testing. On live testing, line your colours up with the conductors you're on. On phase rotation the order genuinely matters, and that's covered with live testing.

Electrician's Guide · Section 7 GN3 · proving dead

Never trust someone else's isolation

✋A colleague or an employer tells you a circuit is isolated and ready for you. You check it yourself. Every time. It doesn't matter who they are. You're the one who'll be working on it, so you're the one at risk.

And if anyone gets annoyed that you want to verify it, that tells you what you need to know about how seriously they take your safety. It runs the other way too — if you tell somebody you've isolated something, you should expect them to check.

EAWR · the duty sits with you

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